Mosaic acoustic transducer for cathode-ray tubes

ABSTRACT

A transducer mosaic includes a plurality of spaced transducers formed with piezoelectric material for converting incident ultrasonic energy into an electrical signal. The piezoelectric is supported on one side by a pressure release barrier, suitably a conductive syntactic foam. The syntactic foam material is a mixture of miniscule hollow glass balls and conductive epoxy resin.

United States Patent Inventor Grant S. Bennett San Mateo, Calif.

Appl. No. 809,683

Filed Mar. 24, 1969 Patented Nov. 23, 1971 Assignee Litton Systems, Inc.

San Carlos, Calif.

MOSAIC ACOUSTIC TRANSDUCER FOR CATHODE-RAY TUBES 15 Claims, 6 DrawingFigs.

US. Cl 313/66, 313/329, 340/10, 313/73, 340/5 MP Int. Cl H0lj 31/49,HO4bl1/00,H01j3l/08 Field of Search 313/65 A,

[56] References Cited UNITED STATES PATENTS 3,325,777 6/1967 Fyler313/66 X 3,013,170 12/1961 Sheldon 313/658 3,321,657 5/1967 Granitsas eta1. 313/73 3,424,932 l/1969 Sheldon 313/73 Primary Examiner-Robert SegalAttorneys-Alan C. Rose, Alfred B. Levine, Ronald W. Reagin and Ronald M.Goldman ABSTRACT: A transducer mosaic includes a plurality of spacedtransducers formed with piezoelectric material for converting incidentultrasonic energy into an electrical signal.

7 The piezoelectric is supported on one .side by a pressure releasebarrier, suitably a conductive syntactic foam. The syntactic foammaterial is a mixture of miniscule hollow glass balls and conductiveepoxy resin.

PATENTEmmv 23 l97l SHEET 1 [IF 2 PATENTEDunv 23 197i 3, 622 825 SHEET 2[1F 2 Shae [4 er; /0 F Form l /arya/z/d/ g l ar/k'd/ f/a/f Arron/varinmmu-v MOSAIC ACOUSTIC TRANSDUCER FOR CATHODE-RAY TUBES This inventionrelates to a transducer mosaic and, more particularly, to a mosaicassembly of ultrasonic transducers by which an ultrasonic energy fieldmay be converted into a corresponding field of electrical voltages foruse in an image conversion tube combination.

Image conversion systems are used to provide a visible display ofvisible and, particularly, invisible field patterns. Aptly defined asthe description of the physical properties of a give region, the fieldpatterns may be formed of electromagnetic energy, such as light,compressional wave energy such as ultrasonic energy, atomic energy, suchas X-ray particles, infrared, electric field, and magnetic fields. Oneparticular system has undergone extensive investigation in recentyearsthe image conversion system for underwater viewing. In such systemsan object submerged in water is illuminated with ultrasonic energy andthe reflected ultrasonic field pattern is detected. With suitabledisplay apparatus, the detected signals are converted into a visibleimage. Other less sophisticated image conversion systems employultrasonic energy in a type of scanning sonarradar arrangement and inthat way construct a visible display of a submerged object. In eachinstance the element vital to such system is the image conversion tubewhich incorporates or has associated therewith acoustic to voltagetransducers of suitable resolution.

The Sokolov-type image conversion tube is well described in theliterature. Its essential element is the use of a solid slab ofpiezoelectric material, typically quartz, which, in addition to itsacoustic electrical conversion properties, has mechanical propertieswhich permit it to withstand the pressure differential between thevacuum of the tube on one side and ambient pressure on the other side.This latter requirement, however, places a size limitation on thecrystal which varies with frequency of the ultrasonic energy used in thesystem and which, in turn, limits resolution. Typically, the resolutionpossible from the Sokolov-type tube is limited to lines per inch.

in the Hydrocon image conversion system described in US Pat. No.3,325,777 a mosaic of transducers are employed. Each transducer isconnected through its own circuit, including amplifiers, detectors, etc.to the wire fiber electronics faceplate of an image conversion tube. Thetube faceplate is a ceramic plate, an insulator, penetrated by manysmall wires so that any externally impressed voltages applied to thewires from the transducer appears at the other end of the wire fiberinside the tube. The voltage distribution so formed is read by anelectron beam within the tube that scans all the wires. Thereafter thisinformation is processed and coupled to a conventional displayapparatus. As is apparent, the resolution available in this system is inone instance limited only by the density of wires, the size of thetransducer, and the number of transducers in the mosaic.

In the Hydrocon system the conversion tube faceplate is an integral partof the image conversion tube and cannot be removed without requiring thetube to be completely rebuilt. This is also true of the Sokolov tube. Asis well known, the individual transducer elements are frequencydependent. To the extent that ultrasonic energy of different frequencyranges are to be employed for illumination" of the submerged object, adifferent conversion tube that tailored for use at those frequenciesmust be provided. This is expensive and cumbersome. An alternativeapproach used in the laboratory which permits the change of transducerfaceplate or front end is to have a removable faceplate, although suchresults in a leaky tube, and to maintain a vacuum pump on the tubecontinuously. Even for use in the laboratory such appears unnecessarilycumbersome and unwieldly.

Previous mosaic designs, such as those used in the Hydrocon systemhereinbefore discussed, have assumed that the material in which thetransducer elements are embedded approximate the acoustic impedance ofthe water and are acoustically absorptive so that it simulates aninfinite medium. Nonetheless as is apparent in the description of USPat. No.

3,325,777, the construction of the mosaic and the manner in which it isattached to the wire fiber optics faceplate of the image conversion tubeis complex and tedious. To the extent that each transducer must beindividually wired to the faceplate an ominous and tedious burden ispresented, especially where a system of good resolution might employthousands of such transducers in a single mosaic.

Therefore, it is an object of this invention to provide a new and usefultransducer mosaic useful for converting energy fields into correspondingfields of electrical signals;

It is a further object of the invention to provide an image conversiontube having the simplicity of the Sokolov tube and resolution capabilityof the Hydrocon tube;

It is an additional object of the invention to provide a transducermosaic of novel and improved construction; and

It is a still further object of the invention to provide a simple andeasily assembled ultrasonic transducer arrangement for an imageconversion tube;

It is an additional object of the invention to provide a new method ofconstructing a transducer mosaic. In accordance with the invention atransducer mosaic incorporates a layer of transducing material atop alayer of pressure release material wherein the pressure release materialis a conductive syntactic foam. In accordance with another aspect of theinvention the foam comprises a mixture of miniscule hollow glass ballsand a conductive epoxy resin, for example, in the proportion by weightof l to 5. Further, the foam layer is joined to a wire fiber faceplateadapted to be incorporated as the front end of an image conversion tube.

The foregoing and other objects of the invention together with otheradvantages and features believed to be characteristic of the inventionboth as to its organization and to its method of operation are betterunderstood from the following detailed description taken together withthe figures of the drawing in which:

FIG. 1 illustrates schematically in perspective an image conversion tubeembodying the invention;

FIGS. 2a, 2b, and 2c illustrate several steps used in the assembly ofatransducer mosaic ofthe invention;

FlG. 3 illustrates schematically in cross section a side view of theupper portion of an image conversion tube embodying the invention; and,

FIG. 4 illustrates a schematic cross section, similar to that of FIG. 3,for an alternative embodiment.

FIG. 1 schematically illustrates the complete image conversion tubewhich includes the transducer mosaic l, fiber optics faceplate 2, andthe tube body 3. The tube body 3 includes conventional elements, notillustrated, ofa pickup device such as the vidicon or orthicon andincludes a source of electrons for forming a beam which is used to scanthe faceplate and is conventional. Reference may be made to US. Pat. No.3,325,777 and to the literature for greater details. Fiber opticsfaceplate 2 is represented by the dashed lines as exemplary since itforms the end of the imaging tube body. The faceplate includes a matrixof very fine wires 4 extending therethrough from top to bottom. Thesewires are insulated from one another by the glass or ceramic material ofthe faceplate.

Mosaic 1 is attached to and supported by faceplate 2. Mosaic 1 consistsofa plurality of insulated piezoelectric elements 5 in the form of athin slab. The piezoelectric slab is attached to and supported by apressure release material 6 which in turn is attached to and supportedby faceplate 2. For purposes of joining the elements as described asuitable adhesive, suitably conductive epoxy is used. The individualtransducers are insulated from one another by a gap or space 8 betweeneach. This gap 8 extends slightly into faceplate 2 and prevents any wirein the faceplate from shorting together any of the individualtransducers. The pressure release barrier 6 is a conductive syntacticfoam. This is a material which has acoustically absorptive or deadeningproperties and is electrically conductive. Accordingly, the pressurerelease barrier provides an electrical path from the bottom side of eachpiezoelectric slab through to one or more wires 4 on the top offaceplate 2. The

faceplate fiber wires provide the electrical path internally of tubebody 3 where any voltages presented thereon are available for electronbeam scanning or readout" in the conventional manner. Not illustrated inthis figure to complete a functional mosaic, a wax or paraffinlikesubstance is filled into the slits 8 or an electrically conductive thinfoil material covers the top of the transducers. In the latter structurethis directly places the entire top side of the individual transducerselectrically in common. In the first mentioned approach the commonconnection is made between the piezoelectric surfaces and the water. Asis apparent, most water has sufficient electrical conductivity due toimpurities for this application. If desired, the foil may be bonded tothe transducer with conductive epoxy or ultrasonic welding techniques.In turn the foil is covered with a thin waterproof material.Alternatively, a thin rubber covering, either electrically conductive orwith a thin electrically conductive coating on its inner side, may beplaced over the top of the transducer array and tube front to accomplishboth functions of waterproofing the transducer and placing the top sideof the piezoelectric material 5 electrically in common. In the waxapproach, the wax maintains insulation between the individualtransducers.

As has been described, the pressure release barrier 6 is a conductivesyntactic foam. More particularly, this consists of a cured mixture ofconductive epoxy resin and miniscule hollow glass balls. A magnifiedview of the material is illustrated at 9. A suitable conductive epoxyresin is manufactured by the Hysol Corporation of California and soldunder the designation K-20. This is a two part epoxy in which silver hasbeen added so that, in the cured form, the epoxy is electricallyconductive. The other ingredient consists of miniscule hollow glassspheres, called Eccospheres. The size of the spheres in a given batchvaries from about 0.0012 to 0.005 inches (30 to 125 microns) and areabout 2-microns wall thickness. This variation is not important andeither larger or smaller spheres as available may be used. Glass spheresas specified in this example are available from Emerson-Cummings Co. andare sold under the designation Eccospheres VT."

A layer of the conductive syntactic foam is manufactured as follows:Approximately 5 parts by weight of the epoxy is thoroughly mixed with lpart by weight of the glass spheres to form a uniform mixture. Themixture is then packed into a mold of the desired shape, taking care toensure the filling of all corners in the mold, and allowed to cure orharden. The syntactic foam body is then removed from the mold. Suitablyit is found that the foam can be ground, cut, and machined for finishingoperations. Additionally, the specific gravity is less than l-itfloats." In addition, the acoustic velocity -in the foam substantiallycorresponds to that of water. Measurements made with an ordinaryohmmeter on the foam of a few lOths of an ohm irrespective of theelectrode placement. While a very high conductivity is not a requisiteor critical factor the cited results are very satisfactory. The slab orlayer of material so formed and cut is at this stage represented as slab10 in FIG. 2a.

The slab of piezoelectric material 12 is suitably Lead Titanate-LeadZirconate. This is available from Channel Industries under thedesignation CH 5400." Piezoelectric layer 12 is then attached to thesurface of the syntactic foam slab 10 with a conductive adhesivematerial 13, suitably the conductive epoxy K-ZO, which provides both afirm mechanical bond and electrical continuity. The thickness of theslabs is a matter of design choice. By way of example the thickness ofthe piezoelectric layer is 0.0I2 inches and the thickness of thesyntactic foam layer is 0.050 inches. Both may be of length and widthofl inch.

As illustrated in FIG. 2b the assembled transducer is secured to thefaceplate 15 of a pickup-type imaging tube. A conductive adhesive 14suitably epoxy K-20, is spread on either the remaining surface of thefoam or the faceplate or both and the foam and faceplate are adhesivelyjoined together. As previously described, faceplate I5 is ofaconventional conductive fiber optic construction and consists;

preferably, of 2 m. electrically conductive wires 16 in 4-mil centers ina glass or ceramic matrix. It is evident that the assembly described inthe alternative may be completed by first securing same to the faceplate15 before the faceplate is assembled to the imaging tube body.

The imaging tube as thus far formed with attached transducers is mountedin a suitable holding fixture. A saw, not illustrated, is then used tocut slots 18 through the layers to obtain the structure of FIG. 3 and aswas illustrated in FIG. 1. The grooves or slots 18 are cut through bothhorizontally and vertically to form a checkerboard or mosaic ofindependent ultrasonic transducers. The cut is preferably of a depth at19 just sufficient to penetrate the fiber optic faceplate 15. The sawused for cutting may be of tungsten carbide as thin as can be obtained.A 0.003 inch thick diamond saw is useful for this purpose.

The front end of the imaging tube is then covered with a thin waterproofrubber sheath 21 having a foil or other conductive coating on its innersurface which places the upper surface of all the piezoelectric elementselectrically in common.

Alternatively, instead of a sheath the slots 18 may be filled with a waxor paraffinlike substance 22 shown in FIG. 4. The wax maintainselectrical insulation between individual transducers etc. in the mosaicwhen the imaging tube is submerged in water and the water, almost alwayscontaining some impurities, and therefore having sufficient electricalconductivity, acts as theelectrical conductor common to the top surfacesof all the piezoelectric slabs. The elements in FIG. 4 similar to thosein FIG. 3 are labeled with the same numerals primed. The wax may beadded after the first parallel rows of slots are cut to providemechanical support during cutting of the rows of slots perpendicular tothe first. The wax is filled into these latter slots and then the uppersurface of the piezoelectric is wiped clean.

It is apparent that an alternative transducer construction in which someofthe rows or columns are electrically in common while beingsignificantly decoupled or isolated acoustically, may be accomplished bysimply cutting the slot separating those rows or columns to a depthnearly but not completely through the syntactic foam layer.

As apparent, the transducer construction described in this applicationmay be made of any size or shape, large or small, as is desired simplyby cutting in the desired manner. Additionally, the materials describedmay be used as a single transducer without cutting. For example, theproperties of the piezoelectric and syntactic foam pressure releasebarrier herein described are useful without an imaging tube and can beused in other combinations to detect ultrasonic energy in water. Theacoustic properties of that combination closely approximate the acousticimpedance of water and therefore make such possible and desirable inother applications.

In operation an ultrasonic energy field is incident upon the sheath, notillustrated, covering the mosaic transducers of FIG. 1. As isconventional, the compressional wave energy distorts or compresses thepiezoelectric 5 which in turn develops a voltage across its top andbottom faces proportional in magnitude to the applied stress. Thevoltages from the bottom surface of the individual transducer faces passacross the attached conductive syntactic foam material 6 to the wirefiber faceplate 2. Depending upon the number of wires 4 in the faceplatecovered by the foam material, the voltages pass through faceplate 2 bymeans of wire or wires 4 to the inside evacuated regions of the imagingtube. The voltages so appearing are scanned or read in the conventionalmanner by the electron beam. As is apparent, because of the groovespacing the transducers, they are isolated from one another bothelectrically and acoustically.

Should it be desired to replace the transducer mosaic attached to theimaging tube faceplate as illustrated in FIG. 1 with another havingperhaps different thicknesses of piezoelectric or foam layers than theoriginal substitution is relatively simple. The piezoelectric andsyntactic foam layers being joined to the faceplate with epoxy aresimply scrapped off and the faceplate is cleaned with a suitable epoxysolvent.

Then a new mosaic is attached to the faceplate in the same manner asdescribed for the bonding of the original. Likewise,

the new material is cut with suitable grooves and of such spacingdesired in the same manner described for the original.

It is to be understood that the abovedescribed arrangements are intendedto be illustrative of the invention and not by way of limitation sincenumerous other arrangements and equivalents suggest themselves to thoseskilled in the art and do not depart from the spirit and scope of myinvention. Accordingly, it is to be expressly understood that theinvention is to be broadly construed within the spirit and scope of theappended claims.

What I claim is:

1. An improved front end for an imaging tube comprising in combination:V

a. A plurality of acoustic transducer means spaced from one another toform a mosaic of transducers, each of said transducer; means forconverting incident acoustic energy into a corresponding electricalsignal;

b. a plate having a top and bottom surface and containing a plurality ofelectrically conductive fibers spaced and insulated from one another toform a mosaic of conductive fibers with each of said fibers extending atleast between said top surface and bottom surface to permit the passageof electrical voltages therebetween;

c. a plurality of spaced coupling means, said plurality corresponding innumber to said plurality of transducer means, each of said couplingmeans physically supporting a corresponding one of said transducer meansand forming an electrically conductive passage between saidcorresponding one of said transducer means and at least onecorresponding conductive fiber in said plate, each of said couplingmeans attached on one side to a corresponding one of said transducermeans and on another side to said plate, and wherein, each of saidcoupling means consists of an electrically conductive syntactic foammaterial.

2. The invention as defined in claim 1 wherein said transducer meanscomprises piezoelectric material for converting acoustic energy incidentthereupon into an electrical signal.

3. The invention as defined in claim 1 wherein said electricallyconductive syntactic foam material consists of a mixture of minutehollow glass balls in a matrix of electrically conductive epoxymaterial.

4. An improved detection device comprising:

a. a mosaic of spaced acoustic transducer means, each of said acoustictransducer means in said mosaic for converting incident acoustic energyinto a corresponding electrical signal;

b. a fiber wire plate;

said fiber wire plate containing a mosaic of electrically conductivewire fibers therethrough spaced and insulated from one another;

d. a mosaic of spaced coupling means each of said coupling meansphysically supporting a corresponding one of said transducer means andforming a electrically conductive passage between one of said transducermeans and at least one conductive wire fibers in said plate each saidcoupling means supporting on one side an individual transducer means andon its other side attached to said plate, and

d2. each said coupling means consisting of an electrically conductivesyntactic foam material; and

e. a sheath covering the outer sides of all said acoustic transducermeans, opposite from the side thereof connected to said coupling means,said sheath having an electrically conductive surface for placing saidouter side of all said material.

6. The invention as is defined in claim 4 wherein said transducer meansis a piezoelectric transducer for converting acoustic energy intocorresponding electrical signals.

7. An image conversion tube containing a fiber wire faceplate, saidfiber wire faceplate containing a mosaic of electrically conductive wirefibers therethrough spaced and insulated from one another; electron beammeans for scanning an end of each of said plurality of conductive wirefibers to monitor the presence and magnitude of electrical potentialsthereon; a mosaic of spaced acoustic transducer means, each of saidtransducer means in said mosaic for converting incident acoustic energyinto a corresponding electrical signal; a mosaic ofspaced couplingmeans, each ofsaid coupling means physically supporting a correspondingone of said transducer means and forming an electrically conductivepassage between one of said transducer means and at least one conductivewire fiber in said faceplate; each said coupling means supporting on oneside an individual transducer means and on its other side attached tosaid faceplate, and each said coupling means consisting of anelectrically conductive syntactic foam material.

8. The invention as defined in claim 7 wherein said transducer means isa piezoelectric transducer for converting acoustic energy intocorresponding electrical signals.

9. The invention as defined in claim 8 wherein said piezoelectrictransducers are thin slabs of piezoelectric material.

10. The invention as defined in claim 7 wherein said conductivesyntactic foam consists substantially of a mixture of minute hollowglass spheres and electrically conductive epoxy material.

11. The invention as defined in claim 10 wherein said mixture contains aportion by weight of epoxy material at least twice as great than theportion of glass spheres.

12. The invention as defined in claim 10 wherein said transducercomprises a piezoelectric material for converting incident acousticenergy into a corresponding electrical voltage.

13. The invention as defined in claim I wherein each transducer in saidmosaic comprises a slab of piezoelectric material.

14. The invention as defined in claim 13 wherein the space betweenadjacent transducer means and between adjacent coupling means comprisesa fill of waxlike material to maintain insulation therebetween when theformer is immersed in an electrically conductive fluid.

15. An improved detection device for use in combination with the wirefiber faceplate of a cathode ray detection tube comprising:

a. piezoelectric transducer means for converting incident acousticsignals into an electrical signal;

b. coupling means having front sides and backsides, said coupling meansattached to and supporting on said front side said transducer means andadapted to be attached on said backside to said wire fiber faceplate forcoupling said electrical signals from said front side to said backside;

c. said coupling means consisting substantially of a mixture of minutehollow glass spheres and electrically conductive epoxy material in whichmixture said epoxy material by proportion is approximately twice asgreat by weight as the portion of said glass spheres; and

d. conductive epoxy means for securing said coupling means to saidtransducer means and for securing coupling means to said wire fiberfaceplate.

W050 UNITED STATES PATENT OFFICE 5 9 CERTIFICATE OF CORRECTION PatentNo. 3, 2, 5 Dated November 23, 1971 Inventor(s) Grant S. Bennett It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby Corrected as shown below:

n Column 6, line LIB, cancel the number "1" and substitute therefor-7--.

Signed and sealed this 16th day of May 1972.

(SEAL) Attes t3:

EDWARD M.FLETCHFR,JR.

"I. ting Officer ROBERT GOTTSCHALK Commissioner of Patents

2. The invention as defined in claim 1 wherein each said transducermeans comprises piezoelectric material for converting acoustic energyincident thereupon into an electrical signal.
 3. The invention asdefined in claim 1 wherein said electrically conductive syntactic foammaterial consists of a mixture of minute hollow glass balls in a matrixof electrically conductive epoxy material.
 4. An improved detectiondevice comprising: a. a mosaic of spaced acoustic transducer means, eachof said acoustic transducer means in said mosaic for converting incidentacoustic energy into a corresponding electrical signal; b. a fiber wireplate; c. said fiber wire plate containing a mosaic of electricallyconductive wire fibers therethrough spaced and insulated from oneanother; d. a mosaic of spaced coupling means each of said couplingmeans physically supporting a corresponding one of said transducer meansand forming a electrically conductive passage between one of saidtransducer means and at least one conductive wire fibers in said plate,d1. each said coupling means supporting on one side an individualtransducer means and on its other side attached to said plate, and d2.each said coupling means consisting of an electrically conductivesyntactic foam material; and e. a sheath covering the outer sides of allsaid acoustic transducer means, opposite from the side thereof connectedto said coupling means, said sheath having an electrically conductivesurface for placing said outer side of all said transducers electricallyin common.
 5. The invention as defined in claim 4 wherein saidelectrically conductive syntactic foam material comprises a mixture ofhollow glass spheres and an electrically conductive epoxy material. 6.The invention as is defined in claim 4 wherein said transducer means isa piezoelectric transducer for converting acoustic energy intocorresponding electrical signals.
 7. An image conversion tube containinga fiber wire faceplate, said fiber wire faceplate containing a mosaic ofelectrically conductive wire fibers therethrough spaced and insulatedfrom one another; electron beam means for scanning an end of each ofsaid plurality of conductive wire fibers to monitor the presence andmagnitude of electrical potentials thereon; a mosaic of spaced acoustictransducer means, each of said transducer means in said mosaic forconverting incident acoustic energy into a corresponding electricalsignal; a mosaic of spaced coupling means, each of said coupling meansphysically supporting a corresponding one of said transducer means andforming an electrically conductive passage between one of saidtransducer means and at least one conductive wire fiber in saidfaceplate; each said coupling means supporting on one side an individualtransducer means and on its other side attached to said faceplate, andeach said coupling means consisting of an electrically conductivesyntactic foam material.
 8. The invention as defined in claim 7 whereinsaid transducer means is a piezoelectric transducer for convertingacoustic energy into corresponding electrical signals.
 9. The inventionas defined in claim 8 wherein said piezoelectric transducers are thinslabs of piezoelectric material.
 10. The invention as defined in claim 7wherein said conductive syntactic foam consists substantially of amixture of minute hollow glass spheres and electrically conductive epoxymaterial.
 11. The invention as defined in claim 10 wherein said mixtureContains a portion by weight of epoxy material at least twice as greatthan the portion of glass spheres.
 12. The invention as defined in claim10 wherein said transducer comprises a piezoelectric material forconverting incident acoustic energy into a corresponding electricalvoltage.
 13. The invention as defined in claim 1 wherein each transducerin said mosaic comprises a slab of piezoelectric material.
 14. Theinvention as defined in claim 13 wherein the space between adjacenttransducer means and between adjacent coupling means comprises a fill ofwaxlike material to maintain insulation therebetween when the former isimmersed in an electrically conductive fluid.
 15. An improved detectiondevice for use in combination with the wire fiber faceplate of a cathoderay detection tube comprising: a. piezoelectric transducer means forconverting incident acoustic signals into an electrical signal; b.coupling means having front sides and backsides, said coupling meansattached to and supporting on said front side said transducer means andadapted to be attached on said backside to said wire fiber faceplate forcoupling said electrical signals from said front side to said backside;c. said coupling means consisting substantially of a mixture of minutehollow glass spheres and electrically conductive epoxy material in whichmixture said epoxy material by proportion is approximately twice asgreat by weight as the portion of said glass spheres; and d. conductiveepoxy means for securing said coupling means to said transducer meansand for securing coupling means to said wire fiber faceplate.